1. Field of the Invention
The present invention relates to nonreciprocal circuit devices and, in particular, to a nonreciprocal circuit device, such as an isolator or a circulator, used in microwave bands.
2. Description of the Related Art
Nonreciprocal circuit devices, such as isolators or circulators, have a characteristic that allows a signal to be transmitted only in a predetermined specific direction and not in the opposite direction. This characteristic is used by, for example, an isolator used in a transmitting circuit of a mobile communication device, such as an automobile phone or a cellular phone.
This type of a nonreciprocal circuit device includes a ferrite having a center electrode, a permanent magnet for applying a direct-current magnetic field thereto, and other components, such as a matching capacitance and a resistor. International Publication No. WO2007-046229 describes a nonreciprocal circuit device in which a first center electrode and a second center electrode are wound around two principal front and back surfaces of a ferrite, the first and second center electrodes being insulated from and intersecting each other and made of a conductive film, to obtain a smaller insertion loss.
However, in the nonreciprocal circuit device described in International Publication No. WO2007-046229, an insulating layer is disposed between the first and second center electrodes made of the conductive film on the principal surfaces of the ferrite (magnetic substance with a firing temperature of 1,350° C.), and the insulating layer is made of non-magnetic material, such as glass, (firing temperature is 1,000° C.). It is difficult to simultaneously fire these elements, so the number of steps in a production process and the cost are increased. For simplifying the production process and reducing the cost, co-firing is useful. However, the structure in which the ferrite is sandwiched between the pair of permanent magnets presents the problem of increasing an insertion loss if the ferrite and the insulating layer are made of exactly the same material.
From the viewpoint of integrally firing a ferrite, Japanese Unexamined Patent Application Publication No. 10-145111 and Japanese Unexamined Patent Application Publication No. 2002-314308 describe laminating and firing ferrites having different saturation magnetization values. However, in the nonreciprocal circuit device described in Japanese Unexamined Patent Application Publication No. 10-145111, the ferrites having a center electrode have the same saturation magnetization value, so the problem of increasing an insertion loss cannot be solved. Japanese Unexamined Patent Application Publication No. 2002-314308 describes increasing saturation magnetization of a ferrite layer adjacent to a permanent magnet and making the magnetic field distribution uniform.